There are a number of polyphase, typically three-phase, measuring instruments which record power consumption without any external measuring devices. Such an instrument is referred to as a "self-contained meter" and is used on 240 volt and 600 volt delta connected systems and also on two-phase network systems. The latter systems are commonly used, for example, in condominium complexes. These instruments have five connection points positioned to engage respective contacts of a support socket connected to the supply and to the load. Four of the connection points are fixed while the fifth may be positioned as required for the particular installation. In operation of the instrument, if the fifth pin is not connected up, the instrument will register less energy than is actually used. After installation, one can readily determine whether the instrument is measuring power, but one cannot easily determine whether the instrument is measuring all the power that is consumed.
It is an object of the present invention to provide a testing device which can be inserted between the instrument and the support socket, the device being operable to indicate the correctness or otherwise of the connections between the detachable instrument and the power lines.